As electronic devices become smaller and lighter, associated printed circuit boards (PCBs) need to be smaller and thinner. To reduce the size of PCBs, trace lines are made finer, and spaces for electrical interconnections on the PCB are reduced. Integration of passive discrete components (such as embedded resistors) into the PCB is also useful in reducing the size of PCBs. Integrating the functionality of passive discrete components into a PCB's laminate substrate, frees up PCB surface area available for discrete components. Consequently, the use of embedded passive components allows for increased device functionality by incorporation of a greater, number of active components without using more PCB surface area. Furthermore, embedding passive components into a PCB not only allows for further miniaturization of electronic devices, but also improves reliability and electrical performance.
One prior art approach to making embedded resistors has been to use resistive foils that are made by depositing a thin film of an electrically resistive material on a copper foil. Metal alloys having nickel (Ni) and chromium (Cr) as major constituents (hereinafter referred to as “nickel-chromium” or “Ni/Cr” alloys) are typically used as the electrically resistive material in forming embedded resistors. One such metal alloy is Ni/Cr/Si/Al. The process for forming an embedded resistor includes: (a) laminating the resistive foil on a dielectric layer, (b) sequentially etching the resistive foil to form resistors, and (c) burying the formed resistors within a multi-layer printed circuit board.
In the above-mentioned sequential etching process, a first selective etching solution is used to remove unwanted copper without attacking the Ni/Cr alloy. A second selective etching solution is then used to remove the unwanted Ni/Cr alloy without attacking the copper. Acidic chromium etching solutions are preferred in etching Ni/Cr alloys.
There are several patents for acidic chromium etching solutions. For example, U.S. Pat. No. 2,230,156 describes a chromium etching solution containing hydrochloric acid and glycol, the glycol having more carbon atoms than hydroxyl groups, and U.S. Pat. No. 2,687,345 describes a chromium etching solution containing calcium chloride and ethylene glycol. Furthermore, U.S. Pat. No. 4,160,691 describes a chromium etching solution containing hydrochloric acid and glycerin. All of these noted solutions are acidic chromium etching solutions, which effectively remove chromium with little or no attack of the copper. Therefore, these prior art etchants may be used to etch a Ni/Cr alloy layer.
It has been observed that the etching rate of solutions of the type heretofore described decreases significantly as the following ratio increases:                               the          ⁢                                           ⁢          surface          ⁢                                           ⁢          area          ⁢                                           ⁢          of          ⁢                                           ⁢          copper                                              exposed          ⁢                                           ⁢          to          ⁢                                           ⁢          the          ⁢                                           ⁢          etching          ⁢                                           ⁢          solution          ⁢                                           ⁢                      (                          C              SA                        )                                                            the          ⁢                                           ⁢          surface          ⁢                                           ⁢          area          ⁢                                           ⁢          of          ⁢                                           ⁢                      Ni            /            Cr                    ⁢                                           ⁢          alloy                                              exposed          ⁢                                           ⁢          to          ⁢                                           ⁢          the          ⁢                                           ⁢          etching          ⁢                                           ⁢          solution          ⁢                                           ⁢                      (                          R              SA                        )                              
This ratio (hereinafter referred to as “the CSA/RSA ratio”) can reach a value wherein etching of the Ni/Cr alloy is inhibited. Moreover, it has been found that some Ni/Cr alloy etching solutions may dissolve treatments (including, but not limited to, adhesion-promoting treatments (e.g., nodular treatments), thermal barrier layer treatments, stain proofing treatments, and resin resistant coating treatments) that are deposited on the copper surface to enhance the peel strength and shelf life of a resistive foil.
The present invention addresses these and other drawbacks of the prior art and provides an etching solution containing thiourea, for etching a resistive layer comprised of a nickel-chromium alloy.